(1) Field of the Invention
The present invention relates to a method and apparatus which uses combined quadrupole and multipole, primarily octupole, magnet sets to increase the uniformity of a beam used in irradiating large targets or surfaces with charged particles in the beam. In particular the present invention relates to the use of the method and apparatus for ion implantation, for charged particle cancer therapy, for determining radiation effects on biological systems and radiation material damage studies, and for heavy ion irradiation of foils to produce microfilters.
(2) Prior Art
A beam of charged particles of large spatial dimensions with uniform cross-sectional or areal densities at a given location is essential in radiation treatments of tumors, as well as for many other applications. A simple method for the preparation of such a beam is to send the beam from an accelerator through a thin scattering target. The multiple scattering of the beam, which arises mainly from the interaction of the projectile with the nuclear charge of the target nuclei, results in an approximately gaussian angular distribution of the particles emerging from the target along the longitudinal axis of the beam (E. Segre, Nuclei and Particles, (W. A. Benjamin, Inc., New York p39-49 (1965)). By moving the position where irradiation is to take place far from the beam spreading target, the radiation field is enlarged and uniformity of dose is obtained at the cost of intensity.
Two methods have been discussed by Koehler, Sneider and Sisterson (A. M. Koehler, R. J. Schneider and J. M. Sisterson, Medical Physics 4, 297 (1977)) to improve the intensity of radiation fields for a given homogeneity. In the first method a narrow beam is swept over a target. A potential problem is the failure of the sweeper, which could result either in injury to the patient or in damage to material. In the second method, a double scattering scheme, one adds an additional beam spreading target downstream from the first target, with the second target appropriately obstructed near the center on the axis. The net result is an improvement of the flatness of the distribution over a certain irradiation area. A calculation using the double scattering system with a variable circular obstruction before the second scattering indicates a uniformity in the measuring plane of 10% with an efficiency for about 32%, which is considered satisfactory for medical treatment. However, when the scatterings occur within a beam transport or analysis system, a potential weakness of the double multiple-scattering method is the enlargement of the effective object size of the system by the second scattering and the increased loss of intensity due to nuclear interactions.